A standard for a wireless local area network (WLAN) technology has been developed as IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard. IEEE 802.11a and IEEE 802.11b use an unlicensed band in 2.4 GHz or 5 GHz, IEEE 802.11b provides transmission speed of 11 Mbps and IEEE 802.11a provides transmission speed of 54 Mbps. IEEE 802.11g provides transmission speed of 54 Mbps in a manner of applying Orthogonal Frequency Division Multiplexing (OFDM) in 2.4 GHz. IEEE 802.11n provides transmission speed of 300 Mbps for 4 spatial streams in a manner of applying Multiple Input Multiple Output-OFDM (MIMO-OFDM). IEEE802.11n supports a channel bandwidth up to 40 MHz. In this case, IEEE802.11n provides transmission speed of 600 Mbps.
Currently, ongoing effort to develop IEEE 802.11af standard has been performed. IEEE 802.11af standard is a standard set to regulate an operation of an unlicensed device in a TV whitespace (TVWS) band.
The TVWS is a frequency band assigned for a TV broadcast and includes a Very High Frequency (VHF) band (54˜60 MHz, 76˜88 MHz, 174˜216 MHz) and an Ultra High Frequency (UHF) band (470˜698 MHz). The TVWS means a frequency band permitted to an unlicensed device to use under a condition that the unlicensed device does not impede a communication of a licensed device operating in a corresponding frequency band. The licensed device may include a TV, a wireless microphone, and the like.
Although operations of all unlicensed devices are permitted on 512˜608 MHz and 614˜698 MHz except several special cases, a communication between fixed devices is only permitted on 54˜60 MHz, 76˜88 MHz, 174˜216 MHz, 470˜512 MHz. A fixed device indicates a device performing a transmission at a fixed position only. In the following description, a white space band includes the aforementioned TVWS, by which the present invention may be non-limited.
The unlicensed device wishing to use the TVWS should provide a protection function for a licensed device. Hence, the unlicensed device should check whether the licensed device occupies a corresponding band before starting a transmission in the TVWS. In particular, a use of the unlicensed device is permitted only when the licensed device is not used in the white space band.
To this end, the unlicensed device should obtain channel list information available in a corresponding region in a manner of accessing a geo-location database (GDB) via the internet or a dedicated network. The GDB is a database configured to store and manage channel usage information, which is dynamically changing in accordance with information on licensed devices registered to the GDB, geographical locations of the licensed devices, and hours of use. Moreover, such a signaling protocol as a common beacon frame and the like, a spectrum sensing mechanism, and the like may be used to solve a coexistence problem between the unlicensed devices using a white space.
In IEEE 802.11 system, a TVWS terminal may indicate an unlicensed device operating using IEEE 802.11 MAC (medium access control) layer and a PHY (physical) layer in a TVWS spectrum. Unless there is a separate explanation in the present specification, a station (STA) indicates the TVWS terminal operating in the TVWS spectrum.
The STA should provide a function of protecting an incumbent user, which corresponds to a user to which a preferential access is permitted, including a licensed user (TV, wireless microphone, and the like). In particular, if a TVWS is in use by the incumbent user, the STA should stop using a corresponding channel. Hence, the STA should operate in an available channel in a manner of searching for an available channel (i.e., a channel not used by a licensed device) capable of being used by an unlicensed device.
A method of searching for an available channel, which is searched by the STA, includes a scheme of performing a spectrum sensing mechanism, a scheme of finding out a TV channel schedule by accessing a GDB, and the like. An energy detection scheme (a scheme of judging a use of an incumbent user if a strength of a reception signal is greater than a prescribed value), a feature detection scheme (a scheme of judging a use of an incumbent user if a digital preamble is detected), and the like may be utilized as the spectrum sensing mechanism. Subsequently, the STA obtains GDB information based on location information of the STA in a manner of accessing the GDB and finds out whether a licensed device uses a channel in the corresponding location. Accessing the GDB and obtaining information should be performed with a frequency sufficient enough to protect the licensed device.
If it is judged that an incumbent user uses a channel immediately adjacent to a currently used channel, a user equipment (or STA) and a base station (or access point (AP)) may protect the incumbent user with a scheme of lowering a transmit power.
For instance, detail contents on an operation in a TVWS may refer to ‘Second Memorandum Opinion and Order’ (FCC, ET Docket No. 10-174, September 2010.) document, ‘IEEE 802.22: The first cognitive radio wireless regional area network standard’ (C. R. Stevenson, G. Chouinard, Z. Lei, W. Hu, S. J. Shellhammer, and W. Caldwell, IEEE Comm. Mag., pp. 130-138, January 2009) document, ‘TVWS PAR and 5C’ (IEEE 802.11-09/0934r05, September 2009.) document, and the like.